Methods and apparatus for cleaning substrates

ABSTRACT

The present invention discloses a method for cleaning substrate without damaging patterned structure on the substrate using ultra/mega sonic device, comprising: applying liquid into a space between a substrate and an ultra/mega sonic device; setting an ultra/mega sonic power supply at frequency f1 and power P1 to drive said ultra/mega sonic device; after micro jet generated by bubble implosion and before said micro jet generated by bubble implosion damaging patterned structure on the substrate, setting said ultra/mega sonic power supply at frequency f2 and power P2 to drive said ultra/mega sonic device; after temperature inside bubble cooling down to a set temperature, setting said ultra/mega sonic power supply at frequency f1 and power P1 again; repeating above steps till the substrate being cleaned.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application claiming priority under 35U.S.C. § 120 to U.S. patent application Ser. No. 16/334,923, entitledMETHODS AND APPARATUS FOR CLEANING SUBSTRATES, filed Mar. 20, 2019,which is a U.S. National Stage Entry under 35 U.S.C. § 371 ofInternational Patent Application No. PCT/CN2016/099428, filed Sep. 20,2016 and titled METHODS AND APPARATUS FOR CLEANING SUBSTRATES, theentire disclosures of which are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention generally relates to method and apparatus forcleaning substrate. More particularly, relates to controlling the bubblecavitation generated by ultra or mega sonic device during the cleaningprocess to achieve a stable or controlled cavitation on the entiresubstrate, which removes fine particles efficiently without damaging thedevice structure on the substrate.

BACKGROUND

Semiconductor devices are manufactured or fabricated on semiconductorwafers using a number of different processing steps to create transistorand interconnection elements. Recently, the transistors are built fromtwo dimensions to three dimensions such as finFET transistors and 3DNAND memory. To electrically connect transistor terminals associatedwith the semiconductor wafer, conductive (e.g., metal) trenches, vias,and the like are formed in dielectric materials as part of thesemiconductor device. The trenches and vias couple electrical signalsand power between transistors, internal circuit of the semiconductordevices, and circuits external to the semiconductor device.

In forming the finFET transistors and interconnection elements on thesemiconductor wafer may undergo, for example, masking, etching, anddeposition processes to form the desired electronic circuitry of thesemiconductor devices. In particular, multiple masking and plasmaetching step can be performed to form a pattern of finFET, 3D NAND flashcell and or recessed areas in a dielectric layer on a semiconductorwafer that serve as fin for the transistor and or trenches and vias forthe interconnection elements. In order to removal particles andcontaminations in fin structure and or trench and via post etching orphoto resist ashing, a wet cleaning step is necessary. Especially, whendevice manufacture node migrating to 14 or 16 nm and beyond, the sidewall loss in fin and or trench and via is crucial for maintaining thecritical dimension. In order to reduce or eliminate the side wall loss,it is important to use moderate, dilute chemicals, or sometimede-ionized water only. However, the dilute chemical or de-ionized waterusually is not efficient to remove the particles in the fin structure,3D NAND hole and or trench and via. Therefore the mechanical force suchas ultra or mega sonic is needed in order to remove those particlesefficiently. Ultra sonic or mega sonic wave will generate bubblecavitation which applies mechanical force to wafer structure, theviolent cavitation such as transit cavitation or micro jet will damagethose patterned structures. To maintain a stable or controlledcavitation is key parameters to control the mechanical force within thedamage limit and at the same time efficiently to remove the particles.In the 3D NAND hole structure, the transit cavitation may not damage thehole structure, but however, the cavitation saturated inside hole willstop or reduce the cleaning effects.

Mega sonic energy coupled with nozzle to clean semiconductor wafer isdisclosed in U.S. Pat. No. 4,326,553. The fluid is pressurized and megasonic energy is applied to the fluid by a mega sonic transducer. Thenozzle is shaped to provide a ribbon-like jet of cleaning fluidvibrating at ultra/mega sonic frequencies for the impingement on thesurface.

A source of energy vibrates an elongated probe which transmits theacoustic energy into the fluid is disclosed in U.S. Pat. No. 6,039,059.In one arrangement, fluid is sprayed onto both sides of a wafer while aprobe is positioned close to an upper side. In another arrangement, ashort probe is positioned with its end surface close to the surface, andthe probe is moved over the surface as wafer rotates.

A source of energy vibrates a rod which rotates around it axis parallelto wafer surface is disclosed in U.S. Pat. No. 6,843,257 B2. The rodsurface is etched to curve groves, such as spiral groove.

It is needed to have a better method for controlling the bubblecavitation generated by ultra or mega sonic device during the cleaningprocess to achieve a stable or controlled cavitation on the entirewafer, which removes fine particles efficiently without damaging thedevice structure on the wafer.

SUMMARY

One method of the present invention is to achieve a damage freeultra/mega-sonic cleaning on a wafer with patterned structure bymaintaining a stable bubble cavitation. The stable bubble cavitation iscontrolled by setting a sonic power supply with power P₁ at a timeinterval shorter than τ₁, and setting the sonic power supply with powerP₂ at a time interval longer than τ₂, and repeat above steps till thewafer is cleaned, where power P₂ is equal to zero or much smaller thanpower P₁, τ₁ is a time interval that the temperature inside bubbleraises to a critical implosion temperature; and τ₂ is a time intervalthat the temperature inside bubble falls down to a temperature muchlower than the critical implosion temperature.

Another method of the present invention is to achieve a damage freeultra/mega sonic cleaning on a wafer with patterned structure bymaintaining a stable bubble cavitation. The stable bubble cavitation iscontrolled by setting a sonic power supply with frequency f₁ at a timeinterval shorter than τ₁, and setting the sonic power supply withfrequency f₂ at a time interval longer than τ₂, and repeat above stepstill the wafer is cleaned, where f₂ is much higher than f₁, better to be2 times or 4 times higher, τ₁ is a time interval that the temperatureinside bubble raises to a critical implosion temperature; and τ₂ is atime interval that the temperature inside bubble falls down to atemperature much lower than the critical implosion temperature.

Another method of the present invention is to achieve a damage freeultra/mega-sonic cleaning on a wafer with patterned structure bymaintaining a stable bubble cavitation with bubble size less than spacein patterned structure. The stable bubble cavitation with bubble sizeless than space in patterned structure is controlled by setting a sonicpower supply at power P₁ for a time interval shorter than τ₁, andsetting the sonic power supply at power P₂ for a time interval longerthan τ₂, and repeat above steps till the wafer is cleaned, where P₂ isequal to zero or much smaller than P₁, τ₁ is a time interval that thebubble size increases to a critical size equal to or larger than thespace in patterned structures; and τ₂ is a time interval that the bubblesize decreases to a value much smaller than the space in patternedstructure.

Another method of the present invention is to achieve a damage freeultra/mega-sonic cleaning on a wafer with patterned structure bymaintaining a stable bubble cavitation with bubble size less than spacein patterned structure. The stable bubble cavitation with bubble sizeless than space in patterned structure is controlled by setting a sonicpower supply with frequency f₁ for a time interval shorter than τ₁, andsetting the sonic power supply with frequency f₂ for a time intervallonger than τ₂, and repeat above steps till the wafer is cleaned, wheref₂ is much higher than f₁, better to be 2 times or 4 times higher, τ₁ isa time interval that the bubble size increases to a critical size equalto or larger than the space in patterned structures; and τ₂ is a timeinterval that the bubble size decreases to a value much smaller than thespace in patterned structure.

A method of the present invention is to achieve a damage freeultra/mega-sonic cleaning on a wafer with patterned structure bymaintaining a controlled transit cavitation. The controlled transitcavitation is controlled by setting a sonic power supply with power P₁at a time interval shorter than and setting the sonic power supply withpower P₂ at a time interval longer than τ₂, and repeat above steps tillthe wafer is cleaned, where power P₂ is equal to zero or much smallerthan power P₁, τ₁ is a time interval that the temperature inside bubbleraises higher than a critical implosion temperature; and τ₂ is a timeinterval that the temperature inside bubble falls down to a temperaturemuch lower than the critical implosion temperature. The controlledtransit cavitation will provide the higher PRE (particle removalefficiency) with minimized damage to patterned structures.

Another method of the present invention is to achieve a damage freeultra/mega sonic cleaning on a wafer with patterned structure bymaintaining controlled transit cavitation. The controlled transitcavitation is controlled by setting a sonic power supply with frequencyf₁ at a time interval shorter than τ₁, and setting the sonic powersupply with frequency f₂ at a time interval longer than τ₂, and repeatabove steps till the wafer is cleaned, where f₂ is much higher than f₁,better to be 2 times or 4 times higher, τ₁ is a time interval that thetemperature inside bubble raises higher than a critical implosiontemperature; and τ₂ is a time interval that the temperature insidebubble falls down to a temperature much lower than the criticalimplosion temperature. The controlled transit cavitation will providethe higher PRE (particle removal efficiency) with minimized damage topatterned structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B depict an exemplary wafer cleaning apparatus usingultra/mega sonic device;

FIGS. 2A-2G depict variety of shape of ultra/mega sonic transducers;

FIG. 3 depicts bubble cavitation during wafer cleaning process;

FIGS. 4A-4B depict a transit cavitation damaging patterned structure ona wafer during cleaning process;

FIGS. 5A-5C depict thermal energy variation inside bubble duringcleaning process;

FIGS. 6A-6C depict an exemplary wafer cleaning method;

FIGS. 7A-7C depict another exemplary wafer cleaning method;

FIGS. 8A-8D depict another exemplary wafer cleaning method;

FIGS. 9A-9D depict another exemplary wafer cleaning method;

FIGS. 10A-10B depict another exemplary wafer cleaning method;

FIGS. 11A-11B depict another exemplary wafer cleaning method;

FIGS. 12A-12B depict another exemplary wafer cleaning method;

FIGS. 13A-13B depict another exemplary wafer cleaning method;

FIGS. 14A-14B depict another exemplary wafer cleaning method;

FIGS. 15A-15C depict a stable cavitation damaging patterned structure ona wafer during cleaning process;

FIG. 16 depicts another exemplary wafer cleaning apparatus usingultra/mega sonic device; and

FIG. 17 depicts an exemplary wafer cleaning apparatus using ultra/megasonic device;

FIGS. 18A-18C depict another exemplary wafer cleaning method;

FIG. 19 depicts another exemplary wafer cleaning method;

FIGS. 20A-20D depict bubble implosion occurs during the cleaning processwithout damage to patterned structures;

FIGS. 21A-21B depict another exemplary wafer cleaning method.

DETAILED DESCRIPTION

FIGS. 1A to 1B show a wafer cleaning apparatus using a ultra/mega sonicdevice. The wafer cleaning apparatus consists of wafer 1010, wafer chuck1014 being rotated by rotation driving mechanism 1016, nozzle 1012delivering cleaning chemicals or de-ionized water 1032, and ultra/megasonic device 1003 and ultra/mega sonic power supply. The ultra/megasonic device 1003 further consists of piezoelectric transducer 1004acoustically coupled to resonator 1008. Transducer 1004 is electricallyexcited such that it vibrates and the resonator 1008 transmits highfrequency sound energy into liquid. The bubble cavitation generated bythe ultra/mega sonic energy oscillates particles on wafer 1010.Contaminants are thus vibrated away from the surfaces of the wafer 1010,and removed from the surfaces through the flowing liquid 1032 suppliedby nozzle 1012.

FIGS. 2A to 2G show top view of ultra/mega sonic devices according tothe present invention. Ultra/mega sonic device 1003 shown in FIG. 1 canbe replaced by different shape of ultra/mega sonic devices 3003, i.e.triangle or pie shape as shown in FIG. 2A, rectangle as shown in FIG.2B, octagon as shown in FIG. 2C, elliptical as shown in FIG. 2D, halfcircle as shown in FIG. 2E, quarter circle as shown in FIG. 2F, andcircle as shown in FIG. 2G.

FIG. 3 shows a bubble cavitation during compression phase. The shape ofbubbler is gradually compressed from a spherical shape A to an appleshape G, finally the bubble reaches to an implosion status I and forms amicro jet. As shown in FIGS. 4A and 4B, the micro jet is very violent(can reaches a few thousands atmospheric pressures and a few thousands °C.), which can damage the fine patterned structure 4034 on thesemiconductor wafer 4010, especially when the feature size t shrinks to70 nm and smaller.

FIGS. 5A to 5C show simplified model of bubble cavitation according tothe present invention. As sonic positive pressure acting on the bubble,the bubble reduces its volume. During this volume shrinking process, thesonic pressure P_(M) did a work to the bubble, and the mechanical workconverts to thermal energy inside the bubble, therefore temperature ofgas and/or vapor inside bubble increases.

The idea gas equation can be expressed as follows:p ₀ v ₀ /T ₀ =pv/T  (1),

where, p₀ is pressure inside bubbler before compression, v₀ initialvolume of bubble before compression, T₀ temperature of gas insidebubbler before compression, p is pressure inside bubbler in compression,v volume of bubble in compression, T temperature of gas inside bubblerin compression.

In order to simplify the calculation, assuming the temperature of gas isno change during the compression or compression is very slow andtemperature increase is cancelled by liquid surrounding the bubble. Sothat the mechanical work w_(m) did by sonic pressure P_(M) during onetime of bubbler compression (from volume N unit to volume 1 unit orcompression ratio=N) can be expressed as follows:

$\begin{matrix}{w_{m} = {{\int_{0}^{{x0} - 1}{pSdx}} = {{\int_{0}^{{x0} - 1}{\left( {{S\left( {x_{0}p_{0}} \right)}/\left( {x_{0^{-}}x} \right)} \right){dx}}} = {{{Sx}_{0}p_{0}{\int_{0}^{{x0} - 1}{{dx}/\left( {x_{0^{-}}x} \right)}}} = {{{{- {Sx}_{0}}p_{0}{\ln\left( {x_{0} - x} \right)}}|_{0}^{{x0} - 1}} = {{Sx}_{0}p_{0}{\ln\left( x_{0} \right)}}}}}}} & (2)\end{matrix}$Where, S is area of cross section of cylinder, x₀ the length of thecylinder, p₀ pressure of gas inside cylinder before compression. Theequation (2) does not consider the factor of temperature increase duringthe compression, so that the actual pressure inside bubble will behigher due to temperature increase. Therefore the actual mechanical workconducted by sonic pressure will be larger than that calculated byequation (2).If assuming all mechanical work did by sonic pressure is partiallyconverted to thermal energy and partially converted mechanical energy ofhigh pressure gas and vapor inside bubble, and such thermal energy isfully contributed to temperature increase of gas inside of bubbler (noenergy transferred to liquid molecules surrounding the bubble), andassuming the mass of gas inside bubble staying constant before and aftercompression, then temperature increase ΔT after one time of compressionof bubble can be expressed in the following formula:ΔT=Q/(mc)=βw _(m)/(mc)=βSx ₀ p ₀ln(x ₀)/(mc)  (3)where, Q is thermal energy converted from mechanical work, β ratio ofthermal energy to total mechanical works did by sonic pressure, m massof gas inside the bubble, c gas specific heat coefficient. Substitutingβ=0.65, S=1E-12 m², x₀=1000 μm=1E-3 m (compression ratio N=1000), p₀=1kg/cm²=1E4 kg/m², m=8.9E-17 kg for hydrogen gas, c=9.9E3 J/(kg ° k) intoequation (3), then ΔT=50.9° k.The temperature T₁ of gas inside bubbler after first time compressioncan be calculated asT ₁ =T ₀ +ΔT=20° C.+50.9° C.=70.9° C.  (4)

When the bubble reaches the minimum size of 1 micron as shown in FIG.5B. At such a high temperature, of cause some liquid moleculessurrounding bubble will evaporate. After then, the sonic pressure becomenegative and bubble starts to increase its size. In this reverseprocess, the hot gas and vapor with pressure P_(G) will do work to thesurrounding liquid surface. At the same time, the sonic pressure P_(M)is pulling bubble to expansion direction as shown in FIG. 5C, thereforethe negative sonic pressure P_(M) also do partial work to thesurrounding liquid too. As the results of the joint efforts, the thermalenergy inside bubble cannot be fully released or converted to mechanicalenergy, therefore the temperature of gas inside bubble cannot cool downto original gas temperature T₀ or the liquid temperature. After thefirst cycle of cavitation finishes, the temperature T₂ of gas in bubblewill be somewhere between T₀ and T₁ as shown in FIG. 6B. Or T₂ can beexpressed asT ₂ =T1−δT=T ₀ +ΔT−δT  (5)

Where δT is temperature decrease after one time of expansion of thebubble, and δT is smaller than ΔT.

When the second cycle of bubble cavitation reaches the minimum bubblesize, the temperature T3 of gas and or vapor inside bubbler will beT3=T2+ΔT=T ₀ +ΔT−δT+ΔT=T ₀+2ΔT−δT  (6)

When the second cycle of bubble cavitation finishes, the temperature T4of gas and/or vapor inside bubbler will beT4=T3−δT=T ₀+2ΔT−δT−δT=T ₀+2ΔT−2δT  (7)

Similarly, when the nth cycle of bubble cavitation reaches the minimumbubble size, the temperature T_(2n-1) of gas and or vapor inside bubblerwill beT _(2n-1) =T ₀ +nΔT−(n−1)δT  (8)

When the nth cycle of bubble cavitation finishes, the temperature T_(2n)of gas and/or vapor inside bubbler will beT _(2n) =T ₀ +nΔT−nδT=T ₀ +n(ΔT−δT)  (9)

As cycle number n of bubble cavitation increase, the temperature of gasand vapor will increase, therefore more molecules on bubble surface willevaporate into inside of bubble 6082 and size of bubble 6082 willincrease too, as shown in FIG. 6C. Finally the temperature inside bubbleduring compression will reach implosion temperature T₁ (normally T₁ isas high as a few thousands ° C.), and violent micro jet 6080 forms asshown in FIG. 6C.

From equation (8), implosion cycle number n_(i) can be written asfollowing:n _(i)=(T ₁ −T ₀ −ΔT)/(ΔT−δT)+1  (10)

From equation (10), implosion time can be written as following:

$\begin{matrix}{\tau_{i} = {{n_{i}t_{1}} = {{t_{1}\left( {{\left( {T_{i} - T_{0} - {\Delta T}} \right)/\left( {{\Delta T} - {\delta T}} \right)} + 1} \right)} = {{n_{i}/f_{i}} = {\left( {{\left( {T_{i} - T_{0} - {\Delta T}} \right)/\left( {{\Delta T} - {\delta T}} \right)} + 1} \right)/f_{1}}}}}} & (11)\end{matrix}$Where, t₁ is cycle period, and f₁ frequency of ultra/mega sonic wave.

According to formulas (10) and (11), implosion cycle number n_(i) andimplosion time τ_(i) can be calculated. Table 1 shows calculatedrelationships among implosion cycle number n_(i), implosion time τ_(i)and (ΔT−δT), assuming Ti=3000° C., ΔT=50.9° C., T₀=20° C., f₁=500 KHz,f₁=1 MHz, and f₁=2 MHz.

TABLE 1 ΔT − δT (° C.) 0.1 1 10 30 50 n_(i) 29018 2903 291 98 59 τ_(i)(ms) 58.036 5.806 0.582 0.196 0.118 f₁ = 500 KHz τ_(i) (ms) 29.018 2.9030.291 0.098 0.059 f₁ = 1 MHz τ_(i) (ms) 14.509 1.451 0.145 0.049 0.029f₁ = 2 MHz

In order to avoid damage to patterned structure on wafer, a stablecavitation must be maintained, and the bubble implosion or micro jetmust be avoided. FIGS. 7A to 7C shows a method to achieve a damage freeultra or mega-sonic cleaning on a wafer with patterned structure bymaintaining a stable bubble cavitation according to the presentinvention. FIG. 7A shows waveform of power supply outputs, and FIG. 7Bshows the temperature curve corresponding to each cycle of cavitation,and FIG. 7C shows the bubble size expansion during each cycle ofcavitation. Operation process steps to avoid bubble implosion accordingto the present invention are disclosed as follows:

-   -   Step 1: Put ultra/mega sonic device adjacent to surface of wafer        or substrate set on a chuck or tank;    -   Step 2: Fill chemical liquid or gas (hydrogen, nitrogen, oxygen,        or CO₂) doped water between wafer and the ultra/mega sonic        device;    -   Step 3: Rotate chuck or oscillate wafer;    -   Step 4: Set power supply at frequency f₁ and power P₁;    -   Step 5: Before temperature of gas and vapor inside bubble        reaches implosion temperature T_(i), (or time reach τ₁<τ_(i) as        being calculated by equation (11)), set power supply output to        zero watts, therefore the temperature of gas inside bubble start        to cool down since the temperature of liquid or water is much        lower than gas temperature.    -   Step 6: After temperature of gas inside bubble decreases to room        temperature T₀ or time (zero power time) reaches τ₂, set power        supply at frequency f₁ and power P₁ again.    -   Step 7: repeat Step 1 to Step 6 until wafer is cleaned.

In step 5, the time τ₁ must be shorter than in order to avoid bubbleimplosion, and τ_(i) can be calculated by using equation (11).

In step 6, the temperature of gas inside bubble is not necessary to becooled down to room temperature or liquid temperature; it can be certaintemperature above room temperature or liquid temperature, but better tobe significantly lower than implosion temperature T_(i).

According to equations 8 and 9, if (ΔT−δT) can be known, the can becalculated. But in general, (ΔT−δT) is not easy to be calculated ormeasured directly. The following method can determine the implosion timeτ_(i) experimentally.

-   -   Step 1: Based on Table 1, choosing 5 different time τ₁ as design        of experiment (DOE) conditions,    -   Step 2: choose time τ₂ at least 10 times of τ₁, better to be 100        times of τ₁ at the first screen test    -   Step 3: fix certain power P₀ to run above five conditions        cleaning on specific patterned structure wafer separately. Here,        P₀ is the power at which the patterned structures on wafer will        be surely damaged when running on continuous mode (non-pulse        mode).    -   Step 4: Inspect the damage status of above five wafers by SEMS        or wafer pattern damage review tool such as AMAT SEM vision or        Hitachi IS3000, and then the implosion time τ_(i) can be located        in certain range.

Step 1 to 4 can be repeated again to narrow down the range of implosiontime τ_(i). After knowing the implosion time τ_(i), the time τ_(i) canbe set at a value smaller than 0.5τ_(i) for safety margin. One exampleof experimental data is described as following.

The patterned structures are 55 nm poly-silicon gate lines. Ultra/megasonic wave frequency was 1 MHz, and ultra/mega-sonic device manufacturedby Prosys was used and operated in a gap oscillation mode (disclosed byPCT/CN2008/073471) for achieving better uniform energy dose within waferand wafer to wafer. Other experimental parameters and final patterndamage data are summarized in Table 2 as follows:

TABLE 2 CO₂ Process Power Number of conc. Time Density Cycle τ₁ τ₂Damage Wafer ID (18 μs/cm) (sec) (Watts/cm2) Number (ms) (ms) Sites #118 60 0.1 2000 2 18 1216 #2 18 60 0.1 100 0.1 0.9 0

It was clear that the τ₁=2 ms (or 2000 cycle number) introduced as manyas 1216 damage sites to patterned structure with 55 nm feature size, butthat the τ₁=0.1 ms (or 100 cycle number) introduced zero (0) damagesites to patterned structure with 55 nm feature size. So that the τ_(i)is some number between 0.1 ms and 2 ms, more detail tests need to bedone to narrow its range. Obviously, the cycle number related to ultraor mega sonic power density and frequency, the larger the power density,the less the cycle number; and the lower the frequency, the less thecycle number. From above experimental results, we can predict that thedamage-free cycle number should be smaller than 2,000, assuming thepower density of ultra or mega sonic wave is larger than 0.1 wattsorcm²,and frequency of ultra or mega sonic wave is equal to or less than 1MHz. If the frequency increases to a range larger than 1 MHz or powerdensity is less than 0.1 watts/cm², it can be predicted that the cyclenumber will increase.

After knowing the time τ₁, then the time τ₂ can be shorten based onsimilar DEO method described above, i.e. fix time τ₁, gradually shortenthe time τ₂ to run DOE till damage on patterned structure beingobserved. As the time τ₂ is shorten, the temperature of gas and or vaporinside bubbler cannot be cooled down enough, which will gradually shiftaverage temperature of gas and vapor inside bubbler up, eventually itwill trigger implosion of bubble. This trigger time is called criticalcooling time. After knowing critical cooling time τ_(c), the time τ₂ canbe set at value larger than 2τ_(c) for the same reason to gain safetymargin.

FIGS. 8A to 8D show another embodiment of wafer cleaning method using aultra/mega sonic device according to the present invention. The methodis similar to that shown in FIG. 7A, except in step 4 setting ultra/megasonic power supply at frequency f₁ and power with changing amplitude ofwaveform. FIG. 8A shows another cleaning method of setting ultra/megasonic power at frequency f₁ and power with increasing amplitude ofwaveform in step 4. FIG. 8B shows another cleaning method of settingultra/mega sonic power supply at frequency f₁ and power with decreasingamplitude of waveform in step 4. FIG. 8C shows another cleaning methodof setting ultra/mega sonic power supply at frequency f₁ and power withdecreasing first and increasing later amplitude of waveform in step 4.FIG. 8D shows further another cleaning method of setting ultra/megasonic power at frequency f₁ and power with increasing first anddecreasing later amplitude of waveform in step 4.

FIGS. 9A to 9D show another embodiment of wafer cleaning method using aultra/mega sonic device according to the present invention. The methodis similar to that shown in FIG. 7A, except in step 4 setting ultra/megasonic power supply at changing frequency. FIG. 9A shows another cleaningmethod of setting ultra/mega sonic power supply at frequency f₁ firstthen frequency f₃ later, f₁ is higher than f₃ in step 4. FIG. 9B showsanother cleaning method of setting ultra/mega sonic power supply atfrequency f₃ first then frequency f₁ later, f₁ is higher than f₃ in step4. FIG. 9C shows another cleaning method of setting ultra/mega sonicpower supply at frequency f₃ first, frequency f₁ later and f₃ last, f₁is higher than f₃ in step 4. FIG. 9D shows another cleaning method ofsetting ultra/mega sonic power supply at frequency f₁ first, frequencyf₃ later and f₁ last, f₁ is higher than f₃ in step 4.

Similar to method shown in FIG. 9C, the ultra/mega sonic power can beset at frequency f₁ first, at frequency f₃ later and at frequency f₄ atlast in step 4, where f₄ is smaller than f₃, and f₃ is smaller than f₁.

Again similar to method shown in FIG. 9C, the ultra/mega sonic power canbe set at frequency f₄ first, at frequency f₃ later and at frequency f₁at last in step 4, where f₄ is smaller than f₃, and f₃ is smaller thanf₁

Again similar to method shown in FIG. 9C, the ultra/mega sonic power canbe set at frequency f₁ first, at frequency f₄ later and at frequency f₃at last in step 4, where f₄ is smaller than f₃, and f₃ is smaller thanf₁.

Again similar to method shown in FIG. 9C, the ultra/mega sonic power canbe set at frequency f₃ first, at frequency f₄ later and at frequency f₁at last in step 4, where f₄ is smaller than f₃, and f₃ is smaller thanf₁.

Again similar to method shown in FIG. 9C, the ultra/mega sonic power canbe set at frequency f₃ first, at frequency f₁ later and at frequency f₄at last in step 4, where f₄ is smaller than f₃, and f₃ is smaller thanf₁.

Again similar to method shown in FIG. 9C, the ultra/mega sonic power canbe set at frequency f₄ first, at frequency f₁ later and at frequency f₃at last in step 4, where f₄ is smaller than f₃, and f₃ is smaller thanf₁.

FIGS. 10A to 10B show another method to achieve a damage freeultra/mega-sonic cleaning on a wafer with patterned structure bymaintaining a stable bubble cavitation in according to the presentinvention. FIG. 10A shows waveform of power supply outputs, and FIG. 10Bshows the temperature curve corresponding to each cycle of cavitation.Operation process steps according to the present invention are disclosedas follows:

-   -   Step 1: Put ultra/mega sonic device adjacent to surface of wafer        or substrate set on a chuck or tank;    -   Step 2: Fill chemical liquid or gas doped water between wafer        and the ultra/mega sonic device;    -   Step 3: Rotate chuck or oscillate wafer;    -   Step 4: Set power supply at frequency f₁ and power P₁;    -   Step 5: Before temperature of gas and vapor inside bubble        reaches implosion temperature T_(i), (total time τ₁ elapes), set        power supply output at frequency f₁ and power P₂, and P₂ is        smaller than P₁. Therefore the temperature of gas inside bubble        start to cool down since the temperature of liquid or water is        much lower than gas temperature.    -   Step 6: After temperature of gas inside bubble decreases to        certain temperature close to room temperature T₀ or time (zero        power time) reach τ₂, set power supply at frequency f₁ and power        P₁ again.    -   Step 7: repeat Step 1 to Step 6 until wafer is cleaned.

In step 6, the temperature of gas inside bubble can not be cooled downto room temperature due to power P₂, there should be a temperaturedifference ΔT₂ existing in later stage of τ₂ time zone, as shown in FIG.10B.

FIGS. 11A to 11B show another embodiment of wafer cleaning method usinga ultra/mega sonic device according to the present invention. The methodis similar to that shown in FIG. 10A, except in step 5 settingultra/mega sonic power at frequency f₂ and power P₂, where f₂ is lowerthan f₁ and P₂ is less than P₁. Since f₂ is lower than f₁, thetemperature of gas or vapor inside bubble increasing faster, thereforethe P2 should be set significantly less than P1, better to be 5 or 10times less in order to reduce temperature of gas and or vapor insidebubble.

FIGS. 12A to 12B show another embodiment of wafer cleaning method usinga ultra/mega sonic device according to the present invention. The methodis similar to that shown in FIG. 10A, except in step 5 settingultra/mega sonic power at frequency f₂ and power P₂, where f₂ is higherthan f₁, and P₂ is equal to P₁.

FIGS. 13A to 13B show another embodiment of wafer cleaning method usinga ultra/mega sonic device according to the present invention. The methodis similar to that shown in FIG. 10A, except in step 5 settingultra/mega sonic power at frequency f₂ and power P₂, where f₂ is higherthan f₁, and P₂ is less than P₁.

FIGS. 14A-14B shows another embodiment of wafer cleaning method using aultra/mega sonic device according to the present invention. The methodis similar to that shown in FIG. 10A, except in step 5 settingultra/mega sonic power at frequency f₂ and power P₂, where f₂ is higherthan f₁, and P₂ is higher than P₁. Since f₂ is higher than f₁, thetemperature of gas or vapor inside bubble increasing slower, thereforethe P2 can be slightly higher than P1, but must make sure thetemperature of gas and vapor inside bubbler decreases in time zone τ₂comparing to temperature zone τ₁, as shown in FIG. 14B

FIGS. 4A and 4B show that patterned structure is damaged by the violentmicro jet. FIGS. 15A and 15B show that the stable cavitation can alsodamage the patterned structure on wafer. As bubble cavitation continues,the temperature of gas and vapor inside bubble increases, therefore sizeof bubble 15046 also increases as shown in FIG. 15A. When size of bubble15048 becomes larger than dimension of space W in patterned structure asshown in FIG. 15B, the expansion force of bubble cavitation can damagethe patterned structure 15034 as shown in FIG. 15C. Another cleaningmethod according to the present invention are disclosed as follows:

-   -   Step 1: Put ultra/mega sonic device adjacent to surface of wafer        or substrate set on a chuck or tank;    -   Step 2: Fill chemical liquid or gas doped water between wafer        and the ultra/mega sonic device;    -   Step 3: Rotate chuck or oscillate wafer;    -   Step 4: Set power supply at frequency f₁ and power P₁;    -   Step 5: Before size of bubble reaches the same dimension of        space Win patterned structures (time τ₁ elapse), set power        supply output to zero watts, therefore the temperature of gas        inside bubble starts to cool down since the temperature of        liquid or water is much lower than gas temperature;    -   Step 6: After temperature of gas inside bubble continues to        reduce (either it reaches room temperature T₀ or time (zero        power time) reach τ₂, set power supply at frequency f₁ power P₁        again;    -   Step 7: repeat Step 1 to Step 6 until wafer is cleaned;

In step 6, the temperature of gas inside bubble is not necessary to becooled down to room temperature, it can be any temperature, but betterto be significantly lower than implosion temperature T_(i). In the step5, bubble size can be slightly larger than dimension of patternedstructures as long as bubble expansion force does not break or damagethe patterned structure. The time τ₁ can be determined experimentally byusing the following method:

-   -   Step 1: Similar to Table 1, choosing 5 different time τ₁ as        design of experiment (DOE) conditions,    -   Step 2: choose time τ₂ at least 10 times of τ₁, better to be 100        times of τ₁ at the first screen test    -   Step 3: fix certain power P₀ to run above five conditions        cleaning on specific patterned structure wafer separately. Here,        P₀ is the power at which the patterned structures on wafer will        be surely damaged when running on continuous mode (non-pulse        mode).    -   Step 4: Inspect the damage status of above five wafers by SEMS        or wafer pattern damage review tool such as AMAT SEM vision or        Hitachi IS3000, and then the damage time τ_(i) can be located in        certain range.

Step 1 to 4 can be repeated again to narrow down the range of damagetime T_(d). After knowing the damage time τ_(d), the time τ₁ can be setat a value smaller than 0.5 τ_(d) for safety margin.

All cleaning methods described from FIG. 7 to FIG. 14 can be applied inor combined with the method described in FIG. 15 .

FIG. 16 shows a wafer cleaning apparatus using a ultra/mega sonicdevice. The wafer cleaning apparatus consists of wafer 16010, waferchuck 16014 being rotated by rotation driving mechanism 16016, nozzle16064 delivering cleaning chemicals or de-ionized water 16060,ultra/mega sonic device 16062 coupled with nozzle 16064, and ultra/megasonic power supply. Ultra/mega sonic wave generated by ultra/mega sonicdevice 16062 is transferred to wafer through chemical or water liquidcolumn 16060. All cleaning methods described from FIG. 7 to FIG. 15 canbe used in cleaning apparatus described in FIG. 16 .

FIG. 17 shows a wafer cleaning apparatus using a ultra/mega sonicdevice. The wafer cleaning apparatus consists of wafers 17010, acleaning tank 17074, a wafer cassette 17076 holding the wafers 17010 andbeing held in the cleaning tank 17074, cleaning chemicals 17070, aultra/mega sonic device 17072 attached to outside wall of the cleaningtank 17074, and a ultra/mega sonic power supply. At least one inletfills the cleaning chemicals 17070 into the cleaning tank 17074 toimmerse the wafers 17010. All cleaning methods described from FIG. 7 toFIG. 15 can be used in cleaning apparatus described in FIG. 17 .

FIGS. 18A to 18C show another embodiment of wafer cleaning method usinga ultra/mega sonic device according to the present invention. The methodis similar to that shown in FIG. 7A, except in Step 5: Beforetemperature of gas and vapor inside bubble reaches implosion temperatureT_(i), (or time reach τ₁<τ_(i) as being calculated by equation (11)),set power supply output to a positive value or negative DC value to holdor stop ultra/mega sonic device to vibrate, therefore the temperature ofgas inside bubble start to cool down since the temperature of liquid orwater is much lower than gas temperature. The positive value of negativevalue can be bigger, equal to or smaller than power P₁.

FIG. 19 shows another embodiment of wafer cleaning method using aultra/mega sonic device according to the present invention. The methodis similar to that shown in FIG. 7A, except in Step 5: Beforetemperature of gas and vapor inside bubble reaches implosion temperatureT_(i), (or time reach τ₁<τ_(i) as being calculated by equation (11)),set power supply output at the frequency same as f₁ with reverse phaseto f₁ to quickly stop the cavitation of bubble. Therefore thetemperature of gas inside bubble start to cool down since thetemperature of liquid or water is much lower than gas temperature. Thepositive value of negative value can be bigger, equal or less than powerP₁. During above operation, the power supply output can be set at afrequency different from frequency f₁ with reverse phase to f₁ in orderto quickly stop the cavitation of bubble.

FIGS. 20A-20D show bubble implosion occurs during the bubble cavitationgenerated by the ultra or mega sonic energy without damage patternedstructures on the wafer. In some case, the patterned structure such asvias 20034 and trenches 20036 on the wafer 20010 has a certainmechanical strength, such as that the size of bubble cavitationgenerated by high frequency sonic power is controlled in a rather smalldimension which is much smaller than the critical dimension of thepatterned structure, therefore, these small bubble implosion forceexerts a very small impact to the patterned structure with largedimension, or that the material characteristic of the patternedstructure can sustain a certain intensity of mechanical force naturally.If the micro jet force generated by the bubble implosion is controlledwithin the intensity that the patterned structures can sustain, thepatterned structures will not be damaged. In addition, the mechanicalforce of the micro jet generated by the bubble implosion contributes tothe particles or residues removal performance inside the patternedstructures of vias and or trenches or on the surface of the wafer, andit also achieves a higher cleaning efficiency. The micro jet with acontrollable intensity, higher than the implosion point yet lower thanthe damage point, is desired in a cleaning process for a better cleaningperformance and efficiency.

FIGS. 21A-21B show that during the sonic power P₁ working on the bubblesat the time of τ₁, the bubble implosion starts to occur when thetemperature of first bubble reaching to its implosion temperature at thepoint of T_(i), and then some bubble implosion continues to occur duringthe temperature increasing from T_(i) to T_(n) (during the time of Δτ),and then turning the sonic power off in the time interval of τ₂, thetemperature of the bubble cooling down from T_(n) to original T₀ by thesurrounding liquid. T_(i) is determined as a threshold of thetemperature for general bubble implosion in the features of vias andtrenches, which triggers the first bubble implosion.

Since thermal transfer is not exactly uninform in the features, more andmore bubble implosion will keep occurring after the temperature reachingto T_(i). The bubble implosion intensity will become higher and higherwhile the implosion temperature T increasing. However, the bubbleimplosion is controlled under the implosion intensity that would resultin the patterned structures damage by controlling the temperature T_(n)below the temperature T_(d) (controlling time of Δτ), wherein T_(r), isthe bubbles maximum temperature obtained by sonic power keeping workingon the bubbles after the cycles of n, and T_(d) is the temperature ofthe accumulation of certain amount bubbles implosion with a highintensity (or power) to result in the patterned structure damage. In acleaning process, the control of bubble implosion intensity is achievedby the control of the time Δt after the first bubble implosion start, soas to achieve a desired cleaning performance and efficiency, and preventthe intensity too high to cause the patterned structure damage.

FIGS. 21A-21B show another embodiment of wafer cleaning method using anultra/mega sonic device according to the present invention. In order toincrease particle removal efficiency (PRE), it is desired to have acontrolled transit cavitation in the mega-sonic cleaning process. Thecontrolled transit cavitation is reached by setting a sonic power supplywith power P₁ at a time interval shorter than τ₁, and setting the sonicpower supply with power P₂ at a time interval longer than τ₂, andrepeating above steps till the wafer is cleaned, where power P₂ is equalto zero or much smaller than power P₁, τ₁ is a time interval that thetemperature inside bubble raises higher than a critical implosiontemperature; and τ₂ is a time interval that the temperature insidebubble falls down to a temperature much lower than the criticalimplosion temperature. Since the controlled transit cavitation will havecertain bubble implosion in the cleaning process, therefore thecontrolled transit cavitation will provide the higher PRE (particleremoval efficiency) with minimized damage to patterned structures. Thecritical implosion temperature is the lowest temperature inside bubble,which will cause the first bubble implosion. In order to furtherincrease PRE, it is needed to further increase temperature of thebubbles, therefore a longer time τ₁ is needed. Also the temperature ofbubble can be increased by shorting the time of τ₂. The frequency ofultra or mega sonic is another parameter to control the level ofimplosion. Normally, the higher the frequency is, the lower level orintensity of the implosion is.

Another embodiment of wafer cleaning method using an ultra/mega sonicdevice to achieve a damage free ultra/mega sonic cleaning on a waferwith patterned structure by maintaining controlled transit cavitation isprovided according to the present invention. The controlled transitcavitation is controlled by setting a sonic power supply with frequencyf₁ at a time interval shorter than τ₁, and setting the sonic powersupply with frequency f₂ at a time interval longer than τ₂, andrepeating above steps till the wafer is cleaned, where f₂ is much higherthan f₁, better to be 2 times or 4 times higher, τ₁ is a time intervalthat the temperature inside bubble raises higher than a criticalimplosion temperature; and τ₂ is a time interval that the temperatureinside bubble falls down to a temperature much lower than the criticalimplosion temperature. The controlled transit cavitation will providethe higher PRE (particle removal efficiency) with minimized damage topatterned structures. The critical implosion temperature is the lowesttemperature inside bubble, which will cause the first bubble implosion.In order to further increase PRE, it is needed to further increasetemperature of the bubbles, therefore a longer time τ₁ is needed. Alsothe temperature of bubble can be increased by shorting the time of τ₂.The frequency of ultra or mega sonic is another parameter to control thelevel of implosion. Normally, the higher the frequency is, the lowerlevel or intensity of the implosion is.

As described above, the present invention provides a method for cleaningsubstrate without damaging patterned structure on the substrate usingultra/mega sonic device. The method comprises: applying liquid into aspace between a substrate and an ultra/mega sonic device; setting anultra/mega sonic power supply at frequency f₁ and power P₁ to drive saidultra/mega sonic device; after micro jet generated by bubble implosionand before said micro jet generated by bubble implosion damagingpatterned structure on the substrate, setting said ultra/mega sonicpower supply at frequency f₂ and power P₂ to drive said ultra/mega sonicdevice; after temperature inside bubble cooling down to a settemperature, setting said ultra/mega sonic power supply at frequency f₁and power P₁ again; repeating above steps till the substrate beingcleaned.

In an embodiment, the present invention provides an apparatus forcleaning substrate using ultra/mega sonic device. The apparatus includesa chuck, an ultra/mega sonic device, at least one nozzle, an ultra/megasonic power supply and a controller. The chuck holds a substrate. Theultra/mega sonic device is positioned adjacent to the substrate. The atleast one nozzle injects chemical liquid on the substrate and a gapbetween the substrate and the ultra/mega sonic device. The controllersets the ultra/mega sonic power supply at frequency f₁ and power P₁ todrive said ultra/mega sonic device; after micro jet generated by bubbleimplosion and before said micro jet generated by bubble implosiondamaging patterned structure on the substrate, setting the ultra/megasonic power supply at frequency f₂ and power P₂ to drive said ultra/megasonic device; after temperature inside bubble cooling down to a settemperature, setting the ultra/mega sonic power supply at frequency f₁and power P₁ again; repeating above steps till the substrate beingcleaned.

In another embodiment, the present invention provides an apparatus forcleaning substrate using ultra/mega sonic device. The apparatus includesa cassette, a tank, an ultra/mega sonic device, at least one inlet, anultra/mega sonic power supply and a controller. The cassette holds atleast one substrate. The tank holds said cassette. The ultra/mega sonicdevice is attached to outside wall of said tank. The at least one inletis used for filling chemical liquid into said tank to immerse saidsubstrate. The controller sets the ultra/mega sonic power supply atfrequency f₁ and power P₁ to drive said ultra/mega sonic device; aftermicro jet generated by bubble implosion and before said micro jetgenerated by bubble implosion damaging patterned structure on thesubstrate, setting said ultra/mega sonic power supply at frequency f₂and power P₂ to drive said ultra/mega sonic device; after temperatureinside bubble cooling down to a set temperature, setting said ultra/megasonic power supply at frequency f₁ and power P₁ again; repeating abovesteps till the substrate being cleaned.

In another embodiment, the present invention provides an apparatus forcleaning substrate using ultra/mega sonic device. The apparatus includesa chuck, an ultra/mega sonic device, a nozzle, an ultra/mega sonic powersupply and a controller. The chuck holds a substrate. The ultra/megasonic device coupled with the nozzle is positioned adjacent to thesubstrate. The nozzle injects chemical liquid on the substrate. Thecontroller sets the ultra/mega sonic power supply at frequency f₁ andpower P₁ to drive said ultra/mega sonic device; after micro jetgenerated by bubble implosion and before said micro jet generated bybubble implosion damaging patterned structure on the substrate, settingsaid ultra/mega sonic power supply at frequency f₂ and power P₂ to drivesaid ultra/mega sonic device; after temperature inside bubble coolingdown to a set temperature, setting said ultra/mega sonic power supply atfrequency f₁ and power P₁ again; repeating above steps till thesubstrate being cleaned.

The embodiments disclosed from FIG. 8 to FIG. 19 can be applied inembodiments disclosed in FIG. 21 .

Generally speaking, an ultra/mega sonic wave with the frequency between0.1 MHz˜10 MHz may be applied to the method disclosed in the presentinvention.

Although the present invention has been described with respect tocertain embodiments, examples, and applications, it will be apparent tothose skilled in the art that various modifications and changes may bemade without departing from the invention.

What is claimed is:
 1. An apparatus for cleaning a semiconductor wafercomprising features of patterned structures, the apparatus comprising: awafer holder configured to hold the semiconductor wafer; an inletconfigured to apply liquid on the semiconductor wafer; a transducerconfigured to deliver acoustic energy to the liquid; a power supply ofthe transducer; and a controller for the power supply comprising atimer, the controller being configured to control the transducer basedon the timer to: deliver acoustic energy to the liquid at a firstfrequency and a first power level for at least a portion of apredetermined first time period, wherein bubble implosion occurs in thefirst time period, and deliver acoustic energy to the liquid at a secondfrequency and a second power level for at least a portion of apredetermined second time period, wherein the controller is configuredto alternately apply the first and second time periods one after anotherfor a predetermined number of cycles, wherein the first and second timeperiods, the first and second power levels, and the first and secondfrequencies are determined such that no feature is damaged as a resultof delivering the acoustic energy; and wherein the controller is furtherconfigured to control bubble implosion under an implosion intensity thatwould result in damages to the patterned structures by controlling Tnbelow Td, wherein Tn is a maximum temperature of the bubbles after thepredetermined number of cycles, and Td is a temperature of the bubblesthat would result in implosion with a high intensity damaging thepatterned structures.
 2. The apparatus of claim 1, wherein the waferholder comprises a rotating chuck.
 3. The apparatus of claim 1, whereinthe wafer holder comprises a cassette submerged in a cleaning tank. 4.The apparatus of claim 1, wherein the inlet comprises a nozzle.
 5. Theapparatus of claim 1, wherein the transducer is connected to the inletand imparts acoustic energy to the liquid flowing through the inlet. 6.The apparatus of claim 1, wherein the second power level is lower thanthe first power level.
 7. The apparatus of claim 6, wherein the secondpower level is zero.
 8. The apparatus of claim 1, wherein the secondfrequency is higher than the first frequency.
 9. The apparatus of claim1, wherein acoustic energy in the second time period is in antiphase toacoustic energy in the first time period.
 10. The apparatus of claim 1,wherein the first frequency is equal to the second frequency, while thefirst power level is higher than the second power level.
 11. Theapparatus of claim 1, wherein the first frequency is higher than thesecond frequency, while the first power level is higher than the secondpower level.
 12. The apparatus of claim 1, wherein the first frequencyis lower than the second frequency, while the first power level is equalto the second power level.
 13. The apparatus of claim 1, wherein thefirst frequency is lower than the second frequency, while the firstpower level is higher than the second power level.
 14. The apparatus ofclaim 1, wherein the first frequency is lower than the second frequency,while the first power is lower than the second power.
 15. The apparatusof claim 1, wherein the acoustic enemy power level rises from the firstpower level during the first time period.
 16. The apparatus of claim 1,wherein the acoustic energy power level falls from the first power levelduring the first time period.
 17. The apparatus of claim 1, wherein theacoustic energy power level both rises and falls from the first powerlevel during the first time period.
 18. The apparatus of claim 1,wherein the acoustic energy frequency changes to a lower value from thefirst frequency during the first time period.
 19. The apparatus of claim1, wherein the acoustic enemy frequency changes to a higher value fromthe first frequency during the first time period.
 20. The apparatus ofclaim 1, wherein the acoustic enemy frequency changes to a higher valuefrom the first frequency and then back to the first frequency during thefirst time period.
 21. The apparatus of claim 1, wherein the acousticenergy frequency changes to a lower value from the first frequency andthen back to the first frequency during the first time period.
 22. Theapparatus of claim 1, wherein the acoustic energy frequency changes fromthe first frequency to a first lower value lower than the firstfrequency, and then to a second lower value lower than the first lowervalue, during the first time period.
 23. The apparatus of claim 1,wherein the acoustic enemy frequency changes from the first frequency toa first higher value higher than the first frequency, and then to asecond higher value higher than the first higher value.
 24. Theapparatus of claim 1, wherein the acoustic energy frequency changes fromthe first frequency to a first lower value lower than the firstfrequency, and then to a second lower value higher than the first lowervalue but lower than the first frequency.
 25. The apparatus of claim 1,wherein the acoustic energy frequency changes from the first frequencyto a lower value lower than the first frequency, and then to a highervalue higher than the first frequency.
 26. The apparatus of claim 1,wherein the acoustic enemy frequency changes from the first frequency toa higher value higher than the first frequency, and then to a lowervalue lower than the first frequency.
 27. The apparatus of claim 1,wherein the acoustic energy frequency changes from the first frequencyto a first higher value higher than the first frequency, and then to asecond higher value higher than the first frequency but lower than thefirst higher value.
 28. The apparatus of claim 1, wherein the secondfrequency is zero and the second power level remains a constant positivevalue during the second time period.
 29. The apparatus of claim 1,wherein the second frequency is zero and the second power level remainsa constant negative value during the second time period.
 30. Theapparatus of claim 1, wherein the features comprise vias or trencheshaving depth to width ratios of at least
 3. 31. The apparatus of claim1, wherein a device manufacturing node of the semiconductor wafer is nomore than 16 nanometers.
 32. The apparatus of claim 1, wherein the waferholder is further configured to rotate the wafer with respect to thetransducer as acoustic energy is delivered.
 33. The apparatus of claim1, wherein the features are not damaged by expansion of bubbles in thefirst time period.
 34. The apparatus of claim 1, wherein temperaturesinside bubbles decrease in the second time period.
 35. The apparatus ofclaim 34, wherein temperatures inside the bubble decrease to near atemperature of said liquid in the second time period.
 36. The apparatusof claim 1, wherein the first time period is shorter than 2,000 times ofa cycle period of the first frequency.
 37. The apparatus of claim 1,wherein the first time period is shorter than((T_(i)−T₀−ΔT)/(ΔT−δT)+1)/f₁, where T_(i) is an implosion temperature,T₀ is a temperature of the liquid, ΔT is a temperature increase afterone time of compression, δT is a temperature decrease after one time ofexpansion, and f1 is the first frequency value.